Within the past few years monofunctional, zeolitic reforming catalysts have become the object of considerable interest. A monofunctional reforming catalyst has little or no acidic catalytic activity, all of its activity for promoting aromatization reactions is provided by a Group VIII metal. The interest in these catalysts is primarily due to their surprisingly high selectivity for dehydrocyclizing paraffins to produce aromatics. See U.S. Pat. Nos. 4,104,320 to Bernard et al; 4,517,306 and 4,447,316 to Buss; and 4,434,331 and 4,435,283 to Buss et al. However, it has also been found that these catalysts are extremely sensitive to sulfur. They are almost totally deactivated by even a small contamination of sulfur. (For example, see U.S. Pat. No. 4,456,527 to Buss et al.) Accordingly, there is a need for a procedure for regenerating sulfur-contaminated monofunctional, zeolitic reforming catalysts containing a Group VIII metal.
A number of regeneration procedures have been proposed for removing sulfur from sulfur-contaminated bifunctional reforming catalysts (bifunctional catalysts have an acidic component, which provides additional catalytic functions of cracking and isomerization). For example, U.S. Pat. No. 2,853,435 granted on Sep. 23, 1958 to Evering et al; 2,892,770 granted on Jun. 30, 1950 to Caley et al; and 3,622,520 granted on Nov. 23, 1971 to Hayes. More recently, regeneration procedures have been disclosed for sulfur-contaminated bimetallic (having two metals, e.g., platinum/rhenium) bifunctional catalysts. For example, see U.S. Pat. Nos. 3,617,523 granted on Nov. 2, 1971 to Spurlock and 4,033,898 granted on Jul. 5, 1977 to Jacobson et al. However, these sulfur regeneration techniques are ineffective for the monofunctional zeolitic reforming catalysts.
Since the possibility of sulfur poisoning of the catalyst cannot be totally eliminated, even with stringent control of sulfur in the feed, the need for a sulfur regeneration procedure specifically for monofunctional zeolite catalysts is apparent.
Several prior art methods for regenerating monofunctional zeolite catalysts have been proposed. They claim to be effective for removing carbon and/or redistributing platinum. However, none of these methods recognizes or addresses deactivation by sulfur contamination or sulfur removal. For example, European Patent Application No. 0 142 352, filed on Nov. 9, 1984 describes a regeneration procedure for deactivated platinum/L zeolite catalysts. The deactivation remedied by this procedure is caused by coke deposition and platinum agglomeration, not by sulfur contamination. In addition, this European Patent application teaches that it is preferred to add some sulfur to the catalyst to reduce cracking. In the application there is a step to burn off carbonaceous matter and an oxychlorination step to redisperse the agglomerated platinum particles.
Another regeneration procedure for a monofunctional reforming catalyst is disclosed in U.S. Pat. No. 4,493,901 granted on Jan. 15, 1985 to Bernard et al. Bernard et al state that the regeneration procedure for coke deactivation may be improved by following an oxygen burn step and oxychlorination step with a hydration step. Sulfur contamination is not mentioned.
Accordingly, since no reference has addressed reactivation after sulfur contamination, there is a need for a regeneration procedure which can restore the activity of monofunctional zeolitic catalysts which have been deactivated in whole or in part by sulfur contamination. This need has now been satisfied by the invention which is summarized and then detailed below.